Clock frequency

Clock frequency

We know this from the ads: "A Pentium 166 MHZ." The 166 MHZ is the clock frequency. Actually, there is a small crystal on the system board. which continually ticks to the CPU at a steady number of clock ticks per second. At each clock tick something happens in the CPU. Thus, the more ticks per second – the more data are processed per second.

The first CPU's worked at a frequency of 4,77 MHZ. Subsequently then, clock frequencies rates rose to 16, 25, 50, 66, 90, 133 and 200 MHZ to the best today, which probably operate at 266 MHZ. Clock frequencies are still being increased. In a few years we will have CPU's operating at 400 and 500 MHZ.
To reach these very high clock frequencies, one has to employ a technique called clock doubling :

Clock doubling in the CPU

The problem with the high clock frequencies is to ensure that other electronic components keep up with the pace. It is rather simple to make data move very fast inside a chip where the print tracks are microscopic. But when we move outside the chip, other problems appear. The other components must be able to keep up with the pace. When the frequency gets too high, the circuit board print tracks start acting as antennae and various forms of "radio noise" appears. Briefly, it becomes expensive to make the rest of the hardware to keep up with these high frequencies.

The solution to this problem was to split the clock frequency in two:
  • A high internal clock frequency, which governs the pace of the CPU.
  • A lower external clock frequency, which governs the pace on the system bus. This is where the CPU exchanges data with RAM and the I/O units.

Intel's 80486DX2 25/50 MHZ was the first chip with clock doubling. It was introduced in 1992 with great potential. For a lower price you could acquire a chip, which provided 90% of the 486DX50 performance. The DX50 runs at 50 MHZ both internally and externally. The DX2 runs at just 25 MHZ on the system bus. This enables lower cost system boards. Also RAM speed demands are much lower.

Clock doubling occurs inside the CPU. If the system board crystal works at 25 MHZ, the CPU will receive a signal every 40 nanosecond (ns). Internally in the CPU, this frequency is doubled to 50 MHZ. Now the clock ticks every 20 ns inside the CPU. This frequency governs all internal transactions, including integer unit, floating point unit, and all memory management unit operations as well as others. The only area still working at 25 MHZ are external data transfers. That is transfers to RAM, BIOS and the I/O ports.

Today the speed problem is in RAM. The ordinary FPM RAM and EDO RAM can function at a
maximum of 66 MHZ (possibly 75 MHZ). Therefore, Pentium and similar CPU's are "clocked up" 2-4 times internally. They work well at high frequencies like 166, 200, 233 and 266 MHZ.

About CPU cache RAM
The CPU must deliver its data at a very high speed. The regular RAM can not keep up with that
speed. Therefore, a special RAM type called cache is used as a buffer - temporary storage. To get
top performance from the CPU, the number of outgoing transactions must be minimized. The more
data transmissions, which can be contained inside the CPU, the better the performance. Therefore,
the 486 was equipped with a built in mathematical co-processor, floating point unit and 8 KB
L1-cache RAM. These two features help minimize the data flow in and out of the CPU.

Cache RAM becomes especially important in clock doubled CPU's, where internal clock frequency is much higher than external. Then the cache RAM enhances the "horsepower" of the CPU, by allowing faster receipt or delivery of data. Beginning with 486 processors, two layers of cache are employed. The fastest cache RAM is inside the CPU. It is called L1 cache. The next layer is the L2 cache, which are small SRAM chips on the system board. See at the illustration below:

Cache overview

L1-cache first appeared in Intel's 80486DX chip:

Today, bigger and better CPU cache is a natural step in the development of new CPU's.

Areas of development

In the following table, you see some of the technologies, which can be improved in the CPU
design. Note that internal means inside the CPU. External speed, etc. refers to features
immediately outside the CPU – on the system board.